Weibing, your question is a very good one. Unfortunately, it covers a lot of territory. Kudos to Kevin Vosper for his scholarly answers. Lame as it may sound, I have to agree with him. Now for my two cents worth.
Remember, English is a living language, so what is the accepted "norm" today will not be tomorrow or the next day. As Kevin pointed out, technology, especially the internet, is changing expectations for both formal and informal writing very quickly. I read recently, (don't remember where) that some university is actually accepting the ICQ form of abbreviations in essays now. That I find very hard to believe, and a lot harder to accept. I've had students turn in papers written this way. No one does it more than once, though!
Speaking is another can of worms entirely. When you are writing, you know the audience you want to attract, and you craft your writing to suit those expectations. Effective speaking, however, has many more variables, and the speaker is quite often not in control of most of them. Yet, there are some constants. Let's say that you, Weibing, are to address your colleagues in a symposium. You are to read a paper that you have researched concerning a particular aspect of your chosen field. You will choose your words with care as you prepare your presentation, and suit your choices to your perceived expectation of your audience. However, immediately after the symposium, you are accosted by mic-waving members of the press who demand that you tell the public at large "in a nutshell" what all the hoopla is about. You will hardly use the same speech in this context, although you are saying basically the same thing.
So, context dictates to a very large extent the formality of our speech. The problem is, that context is constantly changing. This is not something that can easily be taught in a classroom. Sometimes, life experience is about the only way to really learn something. That does not let teachers off trying to explain things as clearly as possible. Good luck to us all.